An example of such a performance-based alternative is to actually explosion test a dust collector, instead of using the back pressure calculations in NFPA 68, to show that it will withstand certain pressure conditions. Using a combination of field testing and full-scale dust-collection laboratory test apparatus, we have performed such tests for explosion vent ducting applications. Our experience has shown that this approach can sometimes yield more accurate real-world performance data than the calculations provided in NFPA 68.
Vent ducting and sizing. For many years explosion vents were sized using simple ratios — i.e., a given dust collector volume needed one square foot of explosion vent area. However, the old formulas no longer apply. Instead, you must use the new design criteria set forth by NFPA 68. Chapters 7–9 provide the calculations for proper sizing of explosion vents, discharge ducts and other components. A reputable dust collector manufacturer will follow the equations for vent sizing in Chapter 8 and will be able to supply a calculations sheet that becomes part of the compliance documentation kept on file.
As far as vent discharge ducts, NFPA 68 stipulates in Chapter 6 that “Vent ducts and nozzles with total lengths of less than one hydraulic diameter shall not require a correction to increase the vent area.” For example, if an explosion vent has a hydraulic diameter of 40 in., you can use a 40-in.-long duct without any added back pressure consequences. However, a duct length above 40 in. (or whatever length is equivalent to the hydraulic diameter) requires stringent calculations to compensate for the estimated increase in back pressure to the collector.
Therefore, when you need longer ducts and the standard calculations cease to apply, you’ll have to work with your supplier to verify Kst values, duct lengths and strengthening requirements. The performance-based design option may come into play when designing collectors for such applications. Longer ducting usually will be needed if a collector must be located inside.
Figure 3 -- Typical installation: Cartridge
That chapter also notes: “To prevent snow and ice accumulation, where the potential exists, and to prevent entry of rainwater and debris, the vent or vent duct shall not be installed in the horizontal position, unless any of the alternative methods in 18.104.22.168.1 are followed.” This means that if you’re using or considering a dust collector with horizontally mounted filter cartridges that have a horizontally mounted explosion vent, you might need to take extra steps to achieve compliance.
The accepted alternative methods of horizontal venting protection are fixed rain hats, weather covers mounted at an angle to shed snow, or deicing provisions such as a heated vent closure. Opting for one of these methods may necessitate additional safety components and testing. For example, if you use a weather cover, the standard says that restraints shall be used and shall be designed and tested to prevent the cover from becoming a free projectile. The other option is to eliminate horizontal venting altogether through use of a dust collector with vertically installed cartridges that use vertically mounted explosion vents.
Also new in NFPA 68 is a section allowing flameless venting inside buildings. Commercially available products in various configurations meet the standard. Flameless venting devices enable you to safely vent an explosion indoors without having any flame escape from the collector. Figure 4 shows a flameless device used to quench explosions. This is a viable option to ducted explosion vents but it’s not recommended for toxic applications, due to the risk of dust being released into the room where venting occurs.
Figure 4 -- Flameless venting:
Here’re some useful questions to ask your suppliers and contractors:
• Is the explosion venting equipment manufactured by a company specializing in this area or is it home-made by the dust collection manufacturer? Either way, ask for documentation proving that the equipment complies with NFPA 68.
• Will the supplier provide a calculations sheet on vent sizing and vent ducting?
• Does the manufacturer have engineering and testing capabilities that allow use of the performance-based design option where needed?
• Can the supplier perform a hazard analysis or recommend a qualified consultant for this task?
• Can the supplier furnish alternative protection technologies such as flameless venting and explosion suppression (see below)?
• Is the installing contractor familiar with NFPA 68? There’s no formal certification for this, so you’ll have to inquire about specific experience and capabilities.
Explosion suppression and other protection methods. Applications where it isn’t feasible to duct an explosion to the outside through a wall or ceiling require an explosion suppression or suppression-isolation system. Such a system may cost more than the dust collector itself.
Suppression methods are covered in a separate document, “NFPA 69: Standard on Explosion Prevention Systems,” which also has been completely revised. Like NFPA 68, it has gone from a guideline to a standard. NFPA 69 extends beyond the scope of explosion venting to address the whole dust-collection system, i.e., inlet and outlet ducting, spark extinguishing systems, and methods for preventing an explosion from traveling back into the building.
Together, the two related standards will significantly impact the future design and cost of collection systems handling combustible dusts.
Lee Morgan is president of Farr Air Pollution Control, Jonesboro, Ark. Tony Supine is the firm’s technical director. E-mail them at email@example.com and firstname.lastname@example.org.